Quality control is very important during the fabrication of a printed circuit board. Although etching processes are precisely prepared and controlled, some defects may appear on a PCB. Defects such as opens or shorts may immediately lead to rejection of the PCB, while micro-semi-cracks or filamentary shorts, poor cleanliness, or geometrical changes in the conducting paths may create hidden faults, which may deteriorate the functionality of the assembled PCB to the point of rendering it useless.
Existing methods for PCB inspection include various optical methods for PCB layer inspection, electrical methods used mainly for final PCB testing, X-ray and thermal methods. A main requirement for all of these methods is high resolution, considering the small width of modern PCB tracks, which starts at about 100 micron.
The most frequently used direct imaging optical methods are based either on visual recognition processes, or on laser methods that rely on fluorescent effects of the plastic substrate. A manual visual method is limited in performance, labor-intensive and prone to human fatigue. Both visual and laser methods have high optical resolution, but are limited in their ability to reveal narrow cracks or filament-type shorts and other invisible flaws, such as salt, residues left over from the etching process, or under-etching characterized by a series of random copper dots and partially conducting surfaces. The principle of detection in most optical systems is based on differences in reflectivity between the non-conducting substrate and the conductive materials (cooper, gold, etc.). As mentioned by Finarov in U.S. Pat. No. 5,333,052, an optical inspection system becomes ineffective, generating many false alarms when the conductive surfaces suffer from colorization due to metal oxidation (which changes color and reflectivity). Laser inspection systems use illumination of the inspected surface and comparison of the difference in fluorescence between the substrate and the conductors, to provide high contrast in the image of the two materials. However, laser-based optical inspection cannot be used to distinguish between two non-fluorescing materials, such as two metal layers, or to image a thin insulating film on a metal layer that does not fluoresce because of its composition or thinness.
Another well-know technique is electrical contact testing (ECT), mostly used for final PCB testing. In certain cases, ECT is also appropriate for PCB layer testing, especially for first article inspection. Conventional ECT techniques for automated PCB testing apply electrical signals through one set of test pins, and measure output signals through another set of test pins. ECT systems are of two main types: “flying probes” and “bed-of-nails”. Both require tight mechanical contact between the test pins and the tested PCB and high mechanical tolerances, as indicated for example by Soiferman in U.S. Pat. No. 5,424,633. A “flying probes” technique is very slow and of low effectiveness. A “bed-of-nail” system for every PCB requires special and unique mechanical jigs with high mechanical precision. Along with the recent developments in high-density PCBs with extremely fine parts and wiring patterns, it has become difficult to correctly position test probes onto the areas to be inspected in the case of inspections using an in-circuit tester. Moreover, when test probes are positioned on a high density PCB, the circuit board might be damaged due to disconnections induced in the wiring pattern, as mentioned by Kawaike et al in U.S. patent application Ser. No. 10/198,739. Additional disadvantages of ECT include its incapability to localize the position of a defect, and its low ability to observe conducting path geometry violations.
X-ray technologies may also be used in some cases to inspect the quality of PCBs, mostly of assembled ones. These technologies require very expensive and complex equipment, also involving safety issues.
Other methods of PCB inspection are based on thermal imaging of the inspected surfaces, e.g. the method mentioned by Spence in U.S. Pat. No. 5,440,566. In general, thermal methods are characterized by low resolution and complex processing.
Non-contact electromagnetic systems and methods for PCB inspection are known. One such contacless testing system (CTS) is disclosed by Soiferman in U.S. Pat. No. 5,424,633. The inspected PCB is placed under an energizing plate connected to an alternating current (AC) signal, and an EM field is generated around the plate. The EM field penetrates through the inspected PCB workpiece, creating a so-called integrated electromagnetic image of the PCB patterns. A set of EM sensors is placed on the other side of the PCB, with an insulation layer placed between sensor and PCB surface. In a coordinate system in which the inspected plane is XY and the depth direction is Z, the EM field signal detected by a sensor represents a pattern of the inspected PCB at the particular XY coordinate where the sensor is located. The main limitation of this system is its low resolution and the impossibility to reveal defects in the Z (depth) direction. In other words, Soiferman's method cannot provide a high resolution 3-dimensional PCB pattern image. Daalmans, in German Patent No. DE19757575 describes an electromagnetic microscope with an eddy current (EC) measuring head, where the corresponding response signal is detected inductively or capacitively via the measuring head resonance circuits. The measuring head has a collection of small planar coils. The main limitation of Daalmans' method and system is the low inductivity of the coils, which requires the use of an excitation signal from a transmitter coil at very high frequencies. This means that the EC penetration depth is very small, and that it is impossible to detect any defects inside the subsurface region.
Goulette et. al. in U.S. Pat. No. 5,006,788 describe a similar method, where a current is applied to a PCB to be inspected in order to generate an electric or magnetic field distribution on the PCB, by connecting all conducting paths to an AC current source. This feature makes the method non-universal and labour-intensive.
Various types of eddy current probes and probe arrays are known. In general, every EC probe has coils wound on a ferromagnetic core, and placed within close proximity to the inspected surface. The ferromagnetic core is used to intensify an induced electromagnetic field flux. A drive coil is used to induce the magnetic field into inspected surface. A sense coil operates to receive current mutually induced by the resultant flux due to the eddy current flow. Any defect in the conducting surface will disrupt the flow of induced current. This disruption is detected by the sense coil and recognized as a defect.
Kawaike et al. above describe a system and method wherein eddy currents are generated on the PCB board to be inspected. Kawaike's detection method has low resolution for distinguishing between faulty and good patterns, and cannot determine the type of defect and its lateral localization in the PCB.
D. Kacprzak et al., in the 6th Int. Workshop on Electromagnetic Non-destructive Evaluation, 2000.6, (hereafter Kacprzak2000) disclose another type of eddy current sensor using a meander driving coil and an air solenoid pick-up coil as a sense coil. The pick-up coil measures a tangential component generated by eddy currents. Their technique is limited by the very small signal generated by the tangential component, which is very difficult to preserve in practice when high scanning velocities are used. Another limitation of the tangential component measurement is the substantial difficulty in inspecting a multilayer PCB. Their results are reported for double-sided boards and conductor widths larger than 200 micrometers, i.e. dimensions much larger than those used in the PCB industry, i.e. not applicable to modern PCBs manufacturing parameters.
In multilayer PCBs, the ability to separate between layers and between layer sides (see below) is important. It is also important to detect the lateral placement of a defect. The common approach in such a case is to use a multi-frequency driving technique, based on fact that the EC penetration depth is inversely proportional to the exciting electromagnetic field frequency. Bilik in USSR Author Certificate AC191196 and USSR Author Certificate AC789730, and Hedengren in U.S. Pat. No. 5,237,27 describe such methods, in which a multi-frequency AC signal is applied to an EC sensor for adjusting the range of penetration depth, thereby adjusting the flaw detection resolution and sensitivity. Another example of using a multi-frequency technique is disclosed by Young et al. in U.S. Pat. No. 5,182,513, in which multi-frequency driving allows the probe sensitivity to be tuned via a proper choice of drive frequencies, thus obtaining improved response concerning flaw detection. All methods mentioned above use multi-frequency driving to change the EC penetration depth in order to improve the data collection sensitivity. The main disadvantage of each of these methods is the requirement to use a wide range of driving frequencies to change the penetration depth.
The resolution of the sensor is also a very important parameter during PCB inspection. A modern PCB track pitch has a spacing of 75–150 micrometer. Therefore, the resolution of the testing sensor becomes very critical. Prior art eddy current probes provide a focus of the magnetic field flux that is commensurable with the diameter of the exciting coil. Some prior art uses an external focusing fixture around the sensor. Taylor in U.S. patent application Ser. No. 09/416,868 proposes a metallic shield disposed around the coil and operative to focus the eddy current field within the structure. Such a fixture focuses the field only within the shield and not inside the inspected structure.
There is therefore a widely recognized need for, and it would be highly advantageous to have an electomagnetic non-contact PCB and PCB layer testing method that does not suffer from the disadvantages mentioned above.